It is already known that during the operation of the internal combustion engine the amount of fuel to be injected can be controlled by an appropriate actuation of the injection valve, for example through the valve lift of its injector needle, the opening time and/or the fuel pressure (rail pressure) in the injection system. However the real injected fuel amount (actual value) is still especially influenced by manufacturing tolerances and ageing influences of the injection valve. This produces deviations from the predetermined setpoint value, which especially with very small injection amounts, as are needed for example in a pre-injection or in a post-injection for heating up an exhaust gas catalytic converter are relatively large. It is especially disadvantageous that the deviation can be different for each individual injection valve.
To compensate for the deviation between the predetermined setpoint value and the actual value of the amount of fuel injected a method has already been proposed in which the uneven running of the internal combustion engine is evaluated. This method is used especially with diesel motors for zero set adaptation.
Furthermore an injection system, especially for a diesel engine, is known from US 2004/0011325 A1, in which the fuel is injected both in a main injection and also additionally in an auxiliary injection in the internal combustion engine. To calculate the amount of fuel for the supplementary injection the fuel pump is first switched on and through activation of the injection valve a fuel injection is generated and after a stabilization phase a first fuel pressure in the fuel rail is measured. Thereafter the injection valve is deactivated, the fuel pump switched off and a second fuel pressure in the fuel rail measured, before the pump is switched on again. From the two pressure values and taking into account the activation signal for the injection valve as well as further operating parameters, a control unit determines a mode, in accordance with which the fuel amount is determined for the supplementary injection. With this system the required fuel amount is designed to be adapted to the operating conditions of the internal combustion engine.
Furthermore a measuring device is known from DE 197 38 722 A1 which is used on an injection valve test bed, in order to measure the injection rate and amount of fuel injected. With this method the pressure increase in a measurement volume is determined and the injected fuel amount is determined from this.